In recent years, electrophotographic apparatuses, which commonly were used in offices, have been used increasingly for personal purposes, and there is a growing demand for technologies that can achieve, e.g., a small size, a high speed, high image quality, or high reliability for those apparatuses. Under such circumstances, a cleanerless process, a tandem color process, and oilless fixing are required along with better maintainability and less ozone emission. The cleanerless process allows residual toner in transfer to be recycled for development without cleaning. The tandem color process enables high-speed output of color images. The oilless fixing can provide clear color images with high glossiness, transmittance, and offset resistance, even if no fixing oil is used to prevent offset during fixing. These functions should be performed simultaneously, and therefore improvements in the toner characteristics as well as the processes are important factors.
For color printers, a color process employing a four-pass system has been put to practical use. In this color process, an image support member (referred to as a photoconductive member in the following) is charged by corona discharge with a charger, and then is exposed to light signals for a latent image of each color to form an electrostatic latent image. The electrostatic latent image is developed by a first color of toner, e.g., yellow toner, to form a visible image. Then, a transfer member charged with the opposite polarity to that of the charged yellow toner comes into contact with the photoconductive member so that the yellow toner image formed on the photoconductive member is transferred. The photoconductive member is cleaned by removing residual toner that has not been transferred, and the development and transfer of the first color toner ends with discharging the photoconductive member. Subsequently, the same operations as those for the yellow toner are repeated for toner of colors such as magenta and cyan. The toner images of the colors are superimposed on the transfer member so as to form a color image. Then, the superimposed toner image is transferred to paper charged with the opposite polarity to that of the toner.
A tandem color process employing the following configuration also has been proposed. A plurality of image forming stations, each of which includes a charger, a photoconductive member, and a developing unit, are arranged in a row. A first transfer process is performed by successively transferring each color of toner to an endless transfer member in contact with the photoconductive members, so that multilayer transfer color toner images are formed on the transfer member. Then, a second transfer process is performed such that the multilayer toner images formed on the transfer member are transferred collectively to a transfer medium such as paper, an overhead projector (OHP) sheet, or the like. Another tandem color process also has been proposed in which toner continuously is transferred directly to the transfer medium without using the transfer member.
In a fixing process for color images, color toner should be melted and mixed to increase the transmittance. A melt failure of the toner may cause light scattering on the surface or the inside of the toner images, and the original color of the toner pigment is affected. Moreover, light does not reach the lower layer of the superimposed images, resulting in poor color reproduction. Therefore, it is essential for the toner to have a complete melt property and transmittance high enough not to reduce the original color. The light transmittance for an OHP sheet also is a necessary property for the color toner.
When color images are formed, toner may adhere to the surface of a fixing roller and cause offset. Therefore, a large amount of oil or the like should be applied to the fixing roller, which makes the handling or configuration of equipment more complicated. Thus, oilless fixing (no oil is used for fixing) is required to provide compact, maintenance-free, and low-cost equipment. To achieve the oilless fixing, e.g., toner in which a release agent (wax) is added to a binder resin with a sharp melt property is being put to practical use.
However, such toner is very prone to a transfer failure or disturbance of the toner images during transfer because of its strong cohesiveness. Therefore, it is difficult to ensure compatibility between transfer and fixing. In the case of two-component development, spent (i.e., a low-melting component of the toner adhering to the surface of a carrier) is likely to occur by heat generated by mechanical collision or friction between the particles or between the particles and the developing unit. This decreases the charging ability of the carrier and interferes with a longer life of the developer.
Japanese patent No. 2801507 (Patent Document 1) discloses a carrier for positively charged toner that is obtained by introducing a fluorine-substituted alkyl group into a silicone resin of the coating layer. JP 2002-23429 A (Patent Document 2) discloses a coating carrier that includes conductive carbon and a cross-linked fluorine modified silicone resin. This coating carrier is considered to have high development ability in a high-speed process and maintain the development ability for a long time. While taking advantage of superior charging characteristics of the silicone resin, the conventional technique uses the fluorine-substituted alkyl group to obtain properties such as slidability, releasability, and repellency, to increase resistance to wearing, peeling, or cracking, and further to prevent spent. However, the resistance to wearing, peeling, or cracking is not sufficient. Moreover, when the negatively charged toner is used, the amount of charge is excessively small, although the positively charged toner may have an appropriate amount of charge. Therefore, the reversely charged toner (positively charged toner) is generated significantly, which leads to fog or toner scattering. Thus, the toner is not suitable for practical use.
Various configurations of toner also have been proposed. It is well-known that toner for electrostatic charge image development used in an electrophotographic method generally includes a resin component (binder resin), a coloring component including a pigment or dye, a plasticizer, a charge control agent, and an additive, if necessary, such as a release agent. As the resin component, natural or synthetic resin may be used alone or in combination.
After the additive is pre-mixed in an appropriate ratio, the mixture is heated and kneaded by thermal melting, pulverized by an air stream collision board system, and classified as fine powders, thus producing a toner base. In this case, the toner base also may be produced by a chemical polymerization method. Then, an additive such as hydrophobic silica is added to the toner base, so that the toner is completed. The single component development typically uses toner only, and the two-component development uses a developer including toner and a carrier of magnetic particles.
Even with pulverization and classification of the conventional kneading and pulverizing processes, the actual particle size can be reduced to only about 8 μm in view of the economic and performance conditions. At present, various methods are considered to produce toner having a smaller particle size. In addition, a method for achieving the oilless fixing also is considered, e.g., by adding a release agent (wax) to a resin with a low softening point during melting and kneading. However, there is a limit to the amount of wax to be added, and increasing the amount of wax can cause problems such as low flowability of the toner, transfer voids, or a fusion of the toner to the photoconductive member.
Therefore, various ways of polymerization different from the kneading and pulverizing processes have been studied as a method for producing toner. For example, toner may be produced by suspension polymerization. However, the particle size distribution is no better than that of the kneading and pulverizing processes, and in many cases further classification is necessary. Moreover, since the toner is almost spherical in shape, the cleaning property is extremely poor when the toner remains on the photoconductive member or the like, and thus the reliability of image quality is reduced.
Toner may be produced by emulsion polymerization including the following steps: preparing an aggregated particle dispersion by forming aggregated particles in a dispersion of at least resin particles; forming adhesive particles by mixing a resin particle dispersion in which resin fine particles are dispersed with the aggregated particle dispersion so that the resin fine particles adhere to the aggregated particles; and heating and fusing the adhesive particles together.
JP 10(1998)-198070 (Patent Document 3) discloses a method for producing toner for electrostatic charge image development. The method includes the following steps: preparing a resin particle dispersion by dispersing resin particles in a surface-active agent having a polarity; preparing a coloring agent particle dispersion by dispersing coloring agent particles in a surface-active agent having a polarity; and preparing a liquid mixture by mixing at least the resin particle dispersion and the coloring agent particle dispersion. According to this method, the surface-active agents included in the liquid mixture have the same polarity, so that reliable toner with excellent charge and color development properties can be produced in a simple and easy manner.
JP 10(1998)-301332 (Patent Document 4) discloses a method for producing toner with an excellent fixing property, color development property, transparency, and color mixing property. According to this method, a release agent includes at least one kind of ester that contains at least one selected from higher alcohol having a carbon number of 12 to 30 and higher fatty acid having a carbon number of 12 to 30, and resin particles include at least two kinds of resin particles with different molecular weights.
As the release agent, e.g., low molecular-weight polyolefins such as polyethylene, polypropylene, and polybutene, silicones, fatty acid amides such as oleamide, erucamide, amide ricinoleate, and amide stearate, vegetable waxes such as carnauba wax, rice wax, candelilla wax, Japan wax, and jojoba oil, animal waxes such as beeswax, mineral/petroleum waxes such as montan wax, ozocerite, ceresin, paraffin wax, microcrystalline wax, and Fischer-Tropsch wax, and modified waxes thereof are disclosed.
However, when the dispersibility of the release agent added is lowered, the toner images melted during fixing are prone to a dull color. This also decreases the pigment dispersibility, and thus the color development property of the toner becomes insufficient. In the subsequent process, when resin fine particles further adhere to the surface of an aggregate, the adhesion of the resin fine particles is unstable due to low dispersibility of the release agent or the like. Moreover, the release agent that once was aggregated with the resin particles is liberated into an aqueous medium. Depending on the polarity or the thermal properties such as a melting point, the release agent may have a considerable effect on aggregation. Further, a specified wax is added in a large amount to achieve the oilless fixing. Therefore, it is difficult to aggregate the wax with the resin particles that differ from the wax in melting point, softening point, and viscoelasticity and to fuse them together uniformly by heating. In particular, the use of a release agent having a predetermined acid value and a functional group may achieve the oilless fixing, reduce fog during development, and improve the transfer efficiency. However, such a release agent prevents uniform mixing and aggregation of the resin particles with pigment particles in an aqueous medium during manufacture. Thus, there is a tendency to increase the presence of release agent or pigment suspended in the aqueous medium.
Patent Document 1: Japanese Patent No. 2801507
Patent Document 2: JP 2002-23429 A
Patent Document 3: JP 10(1998)-198070 A
Patent Document 4: JP 10(1998)-301332 A